A first chamber has a plurality of first ventilation holes of a same size arranged on extended lines of a plurality of head arrays respectively. A second chamber faces the first chamber across the plurality of head arrays and has a plurality of second ventilation holes of a same size facing the plurality of first ventilation holes. Cross-sectional areas of an internal air flow passage in the first chamber at positions of the first ventilation holes decrease with an increase in a distance of the first ventilation holes from a first end of the first chamber. Cross-sectional areas of an internal air flow passage in the second chamber at positions of the second ventilation holes decrease with an increase in a distance of the second ventilation holes from a second end of the second chamber on the same side of the first end.